It depends what sort of flights you choose. A charter airline will reduce it prices as the departure date draws closer in order to fill any empty seats. However, a scheduled airline - Varig for example - will raise its prices. It is likely that you'll be flying with a scheduled carrier, so the sooner you book, the better. Search for flights on Expedia.co.uk to find out which the cheapest airline is and then book on that airline's website to avoid paying commission.